Host devices can use expansion cards to add functionality over an expansion bus, such as a Peripheral Component Interconnect Express (PCIe) bus. With hot-pluggable cards, the operating system of the host can adapt to removal and/or insertion of cards without requiring a system restart. In one implementation of a hot-plug standard, an attention button is pressed when a user wants to remove a powered PCIe card. Pressing the attention button alerts the operating system of the host that the card will be removed, and allows the operating system to smoothly transition the functions of the card before it is removed. Additionally, a PCIe card may include a Manually-operated Retention Latch (MRL) which must be opened to remove the card. Removing the MRL sends a signal to the host to ensure that the host is prepared for the card to be removed.
More complicated system architectures may involve a host connecting to multiple cards. A PCIe switch may be used to expand the PCIe interface and allow a host to connect to multiple PCIe cards. Additionally, multiple hosts may connect to the same card by changing the Basic Input-Output System (BIOS) of the hosts. Multiple hosts can be connected to PCIe switches, but a dedicated PCIe chip that supports Multi-Root Input/Output Virtualization (MR-IOV) is required.